Reducing flammability of magnesium metal



3,074,812 REBUCENG FLAMMABILETY @1 3 MAGNEE liUh l METAL David M. Young and Norman W. Meyers, Sarnia, ntario, Canada, assignors to The Dow Chemical Company, Midland, ll iieln, a corporation of Delaware No Drawing. Filed Apr. 13, 1959, Ser. No. 3d5a7 3 Claims. (Cl. 1l7--Itl9.ti}

The invention is directed to a method of inhibiting the fianniability of magnesium metal and of magnesiumbase alloys, i.e., alloys containing at least about 85 percent by weight of magnesium. In the interests of simplified expression, such magnesium-base alloys will be con sidered as included herein under the general expression magnesium metal.

Although magnesium metal plate, extrudes, castings, and other shapes of substantial thickness and articles fabricated of such magnesium metal are not highly flammable, thin metal articles, e.g., sheet, wire, and ribbon are flammable at temperatures at which they might be employed. Flammability of such thin or fine magnesium metal articles has been a deterrent to the use thereof where high temperatures are encountered or open flames are present and particularly where direct contact of the magnesium metal with the open flame is likely.

Attempted methods of inhibiting the flammability of metal have not been significantly satisfactory to warrant the adoption of any of such methods for practical purposes.

A desideratum, therefore, exists for an effective method 0; inhibiting the flammability of sheets, wire, ribbons, and the like composed of magnesium metal.

'ihe invention hereinafter described satisfies this desideratum to a highly utilitarian extent. The manner by which the invention is practiced is set out in the ensuing description and is particularly pointed out and defined in appended claims.

The invention consists of a method of lessening the tendency of ma .esium metal and magnesium-base alloys to oxidation and especially to burning by applying to the exposed surfaces thereof (1) an aqueous solution of orthcboric acid or (2) a boron compound capable of yielding orthoboric acid in the presence of water or water vapor, drying the film thus formed, and thereafter baking the thus dried film to form a hard firm film protective of the surface against oxidation.

illustrative of boron compounds capable of yielding orthoboric acid in the presence of water or water vapor and useful in the practice of the invention are borid oxide, boron halides, e.g., BClg, boric acids such as metaboric, pyroboric, and tetraboric acid, inorganic salts of a boric acid, e.g., borax, and lower alkyl borates e.g., methyl borate.

The preferred embodiments of the invention employ one of the following treating compositions (1) an aqueous solution of orthoboric acid, (2) methyl borate alone or as the azeotropic mixture thereof with a lower aliphatic alcohol, e.g., methanol, (3) borax in a slightly acidified aqueous solution, e. g., one having a pH value below 7 but not less than 4 and (4) vaporous or liquid BCl For use of boron compounds which hydrolyze rapidly in water, such as the alkyl borates and boron halides, a ueous solutions thereof are not recommended but the me hod of first contacting the surface of the magnesium metal to be protected with the boron compound in the non aqueous state followed by subjection of the thus contscted surface to moist air is followed.

Before applying the aqueous boron solution employed in the practice of the invention, the magnesium metal or magnesium alloy to be coated or protected must have a r in g suitably clean surface. If cleaning of the magnesium metal is necessary, washing with water containing a commoo detergent provides a satisfactorily clean surface.

The preferred method of applying the boron compound as an aqueous solution is by spraying or brushing it on the magnesium metal piece or article to be proteeter]. Non-aqueous solutions such as the methyl borate may be s rayed or brushed on, as in the use of an aqueous solution, but is usually applied by dipping or immersing the piece to be protected in the liquid methyl borate and thereafter exposing the thus treated surface to moist When a boron halide is employed, the halide is preferably applied as a gas, preferably in a substantially dry atmosphere and, similarly as in the use of the methyl borate, the gas treated surfaces are subsequently exposed to water vapor, the moisture commonly contained in the atmosphere being sufficient.

The magnesium piece or article, e.g., a ribbon, having the boron compound applied to the surface thereof, as true described, is subjected successively to a drying step and a baking step, as stated hereinabove. The drying temmay be between room temperature and a temperature below 200 C. The baking temperature is between 2QG C. and the melting point of magnesium metal or of the alloy thereof being treated. Since the tem perature concerned is that of the film on the surface of the magnesium metal article, the temperature of the ambient atmosphere, e.g., in a warm air oven, may be substantially above 206 C. during the drying time due to the retarding effect on temperature rise of the specific heat of the metal and the evaporation of the moisture leaving the surface. Therefore, both the drying and baking steps may be carried on in the same oven at the same oven temperature of say 269 to 250 C. or even higher. it is customary, however, either to dry the freshly treated article at room temperature or in an oven at about C. and thereafter bake the dried film by either raising the oven temperature or transferring the dried article to an oven having temperature of at least 200 C., a temperature of 253 to 408 C. usually being used, and a temperature of about 300 C. being preferred. Baking should be for a minimum time of 10 minutes. Fifteen minutes is usually employed. As long as 1 or 2 hours may be employed but is not recommended.

During drying and baking, best results are obtained by maintaining the article being treated in a substantially horizontal position or gradually rotating it in such position in the drying and baking atmosphere. For simple or uniform shapes, the article may be stationary. However, for an intricate or complex shaped article, it is recornrnended that it be rotated slowly and preferably also turned end-to-end during the drying operation. A rotation of about once per minute and an end-to-end change of once every three or four minutes is quite satisfactory. During the latter part of the drying period, the rotation may be stopped, if desired, and the article be permitted to remain stationary during the baking period.

In practicing the invention, the spraying or dipping step may be limited to but one application of the boron compound followed by drying and baking, or the spraying or dipping step followed by the drying step may be repeated as many times as desired to build up a film of satisfactory thickness. The baking step may also be repeated as desired but is not essential to this mode of practicing the invention so long as the final film is baked.

When an aqueous solution of the boron compound, e.g., orthoboric acid, is employed, the concentration is not highly critical. Less than 0.5 percent by weight is not recommended but any concentration between 0.5 percent and the saturation point may be employed. The thickness of the coating applied to the magnesium metal article to be protected depends upon a number of factors among which are the concentration and viscosity of the aqueous solution being employed, the contact time, manner of application, and upon the number of applications of the aqueous boron solution. Any continuous thickness of the coating oifers some protection but it is recommended that the aqueous solution be applied in an amount sufficient to produce at least 0.01 milligram of the dried coating per square centimeter of surface covered. To insure better protection, it is recommended that, where the coated magnesium metal, piece or article is to be exposed to particularly high temperatures or to an open flame, the coating solution be applied in sufficient amount to provide at least 0.05 milligram per square centimeter of the surface coated. This is usually best attained by repeated dipping or brushing and drying.

Satisfactory films have been applied to magnesium metal sheet in accordance with the invention in an amount sufliciently thick to provide a weight of about 3 milligrams per square centimeter, of film. An excess of l milligram per square centimeter, however, appears not to be economically feasible. Best results were obtained when the coated magnesium metal article was first dried at a temperature below 200 C. and thereafter baked at a temperature between 300 and 350 C. Coatings which are dried and baked at these respective temperatures offer greater resistance to ignition by an open flame and are more resistant to removal by water-washing.

The following examples are illustrative of the practice of the invention:

EXAMPLE 1 A number of magnesium metal plates, 6" x 6 x 0.064", were cleaned by washing in water containing a common detergent, and then dried and weighed. A saturated aqueous solution or" orthoboric acid was applied to both sides of each plate in a thin layer by means of a paint brush. A substantial portion of the water was allowed to evapcrate at room temperature; thereafter the plates were dried at 110 C. This process was repeated a varying number of times for each of the plates in order to build up films of different thicknesses for purposes of comparison. The thus dried plates were then baked in an oven at 300 C. for an hour and reweighed. The weight of the film thus calculated on the surfaces of each of the plates is set out in milligrams per square centimeter in Table I.

The protection against flammability afforded by the film thus provided was tested as follows: each plate was placed horizontally on a firebrick support which contacted only the corners of each plate. A propane-oxygen flame was llowed to impinge on the center of the plate and the time taken for the plate to burst into flame ascertained. For the purposes of comparison, untreated plates of mag nesium of the same composition and size were subjected to the same test. To insure uniformity of the intensity of heat provided by the propane-oxygen flame, the pro pane-oxygen feed to the burner was continuously monitored. The ignition times in seconds required for the magnesium sheets to burst into flame are set forth in Table I below. Table I Film thickness in mg./cm.

ignition time in seconds Untreated EXAMPLE 2 To show the effect on ignition temperature of magnesium metal ribbon by treating it with a saturated aqueous solution of orthoboric acid in accordance With the invention, in contrast to untreated ribbon, the following comparison was made.

Untreated ribbon was held in the hot portion of a Bunsen burner where the temperature was estimated to be over 850 C. The untreated ribbon burst into flame in 2 seconds. Another strip of the thin ribbon was t d according to the invention by applying to the ribbon a saturated aqueous solution of orthoboric acid, drying by a to and fro motion over a Bunsen burner, and thereafter baking at 300 C. for an hour in accordance with the invention. The weight of the film applied thereto was between 1 and 1.2 milligrams per square centimeter of surface. The magnesium metal ribbon thus coated was then tested similarly to the untreated ribbon by the Bunsen burner test and the time required for the ribbon to burst into flame was recorded. The ribbon treated according to the invention melted and then burst into flame after a period of 250 seconds of time. The effectiveness of the method of the invention is clearly shown by the longer time required for the treated magnesium metal ribbon to burst into flame in contrast to that for the untreated ribbon.

EXAMPLE 3 To determine the resistance of the film applied in accordance with the invention to its removal by contact with water, a series of magnesium ribbon sections was washed in water containing a common household detergent, water-rinsed, dried, and weighed. They were thereafter coated with a film of aqueous orthoboric acid by brush application in a manner similar to Example 2 above. The thus treated ribbons were dried as in Exampie 2 and then placed in an oven at the temperatures and for the periods of time set out in Table II. The thus coated ribbons were again weighed to determine the film weight and thereafter washed, with agitation, for 20 minutes, dried at 130 for 15 minutes, and reweighed. The extent to which the film was removed by the contact with Water during agitation is also set out in Table ll below.

Table II Effect of temperature and time of heat treatment on the water solubility of the protective film] Weight Weight Average Heating Initial Lost Lost Weight Temperature, 'linlo, Wt. of uring During Lost Dur- 0. Hours Film Washing Washing ing men/cm. in in Washing org/cm. percent in percent Room Temperature None 1. 54 l. 62 105 1 108 2. 34 2. G3 112 2. 0S 2. 16 105 1. 59 1. 70 107 1. 93 2. 16 112 2. 71 2. 94 108 130 1. 0S 0. 96 89 77 1. '54 0. 45 33 O. 72 0. 34 47 300 Z O. 78 0. 21 27 21 1 High percent loss of weight is likely due to loss of some magnesium, likely as magnesium compound during washing,

An evaluation of the results of Table II shows that substantial percentages of the coatings are removed when the baking temperature is not over 130 C. It also shows that the percent weight loss is substantially reduced at a baking temperature of 200 C. and a relatively small percentage is removed when the baking temperature is at least 300 C. It further shows that an increase in the temperature up to 400 C. does not result in a substantially more resist-ant coating. It further shows that a baking time of hour produces a coating providing adequate resistance to removal by water.

Aqueous solutions of other boron compounds or substantially non-aqueous boron compounds capable of yielding orthoboric acid in the presence of water or water vapor, simultaneously or subsequently provided, may be substituted for the orthoboric acid with comparable beneficial results.

A number of advantages stem from the practice of the invention among which are added safety in storing, shipping, and handling magnesium articles, e.g., ribbon, fine wire, sheet, foil, and plate. The coating provided in accordance with the invention imparts a water-resistant property to the magnesium metal or magnesium alloy piece thus treated. The treating solution employed is one that may he obtained readily at very low cost. The treating operations are usually simple and may be carried on by unskilled personnel employing simple apparatus. The practice of the invention lends itself readily to a continuous operation whereby the magnesium or magnesium-base metal articles to be coated are passed along on a conveyor means and subjected to a spray of an aqueous solution of a boron compound, e.g. orthoboric acid spray, and .thence carried on into drying and baking ovens.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The method of lessening the flammability of an article consisting of at least percent magnesium by weight which comprises repeatedly coating said article with an aqueous solution containing at least 0.5 percent by weight of orthoboric acid and drying at a temperature belew 200 C. until the dried coating is sufliciently thick to weigh at least 0.01 milligram per square centimeter of surface, and thereafter baking the coating thus formed at a temperature between 250 and 400 C. for at least 10 minutes.

2. A metal article consisting of at least 85% magnesium having a coating formed thereon according to the method of claim 1.

6. The coated article of claim 2 wherein the coating is of a weight of between 0.1 and 3.0 milligrams per square centimeter.

References Cited in the file of this patent UNITED STATES PATENTS 2,200,850 M-iserentino May 14, 1940 2,488,587 Dreyfus et a1 Nov. 22, 1949 2,658,834 Rex Nov. 10, 1953 2,785,091 Rex Mar. 12, 1957 2,875,044 Dunn et al Feb. 24, 1959 2,885,315 Milliken May 5, 1959 FOREIGN PATENTS 826,027 Great Britain Dec. 23, 1959 OTHER REFERENCES Hackhs Chemical Dictionary, second edition, 1937, page 153.

Ser. No. 725,750, De Long (A.P.C.), published Aug. 30, 1949, p. 1432. 

1. THE METHOD OF LESSENING THE FLAMMABILITY OF AN ARTICLE CONSISTING OF AT LEAST 85 PERCENT MAGNESIUM BY WEIGHT WHICH COMPRISES REPEATEDLY COATING SAID ARTICLE WITH AN AQUEOUS SOLUTION CONTAINING AT LEAST 0.5 PERCENT BY WEIGHT OF ORTHOBORIC ACID AND DRYING AT A TEMPERATURE BELOW 200*C, UNTIL THE DRIED COATING IS SUFFICIENTLY THICK TO WEIGH AT LEAST 0.01 MILLIGRAM PER SQUARE CENTIMETER OF SURFACE, AND THEREAFTER BAKING THE COATING THUS FORMED AT A TEMPERATURE BETWEEN 250* AND 400* C, FOR AT LEAST 10 MINUTES. 